This invention relates to a process for producing middle distillate from heavy distillate oil. More particularly, it relates to a staged process wherein a heavy distillate oil containing nitrogenous hydrocarbon impurities is effectively converted to middle distillate using selected catalyst and conditions.
A strong continuing need and demand for middle distillate oil is being felt in the industry. Heavy distillate oil has been and remains a desirable source of middle distillate oil. However, heavy oils are, in large part, diverted to fuel oils because of the lack of an effective method for converting them to lower-boiling products. These oils usually contain significant amounts of nitrogenous hydrocarbon impurities. Thus, in addition to hydrocracking, their conversion involves nitrogen removal, and therein lies a problem. Catalysts normally selective for converting middle distillates to gasoline, for example composites of amorphous silica-alumina cracking base and a hydrogenating component, produce large amounts of dry gases, butanes and gasoline when used with heavy distillate feedstocks (see, for example, U.S. Pat. No. 3,513,086). Conditions satisfactory for selective nitrogen removal by most conventional catalysts are usually unsatisfactory for selective hydrocracking.
It is known to convert a heavy distillate oil to a middle oil using a sulfided nickel-tungsten catalyst composited with a siliceous cracking base having a cracking activity above 45, for example as in U.S. Pat. No. 3,268,437. However, the product appears to only be suitable for use as a cutter oil for residual fuels.
It is also known to convert petroleum distillate to gasoline, middle distillates and isobutane in a two-stage process using (1) in the first stage a weakly acidic hydrocracking catalyst, for example a catalyst containing a Group VI and VIII hydrogenating component on a silica-magnesia support (also see U.S. Pat. No. 3,172,838 and U.S. Pat. No. 3,180,817) and (2) in the second stage an active acidic hydrocracking catalyst. However, (1) silica-magnesia based catalysts usually exhibit excessive fouling rates and (2) the use of an active acidic hydrocracking catalyst in the second stage may promote overcracking of feedstock, thereby favoring gasoline and light hydrocarbon production over desired middle distillate production.
It is further known, for example in U.S. Pat. No. 3,184,402, to maximize middle distillate production from a hydrocarbon distillate in a two-stage process using (1) in the first stage a weakly acidic hydrocracking catalyst, for example Ni-Mo on alumina (also see U.S. Pat. No. 3,513,086), and (2) in the second stage a catalyst comprising a hydrogenating-dehydrogenating component on an active cracking component. However, in addition to requiring use of different catalysts for each stage, yields of middle distillate, based upon feed to the first stage, are only nominal, that is, of the order of 38 to 46 liquid volume percent.
It is an object of this invention to provide an effective and improved process for producing middle distillate from a heavy distillate oil containing nitrogenous hydrocarbon impurities.
Other objects will be clear from the description and examples herein.